What causes ice-ages?
Fluctuations in the amount of insolation (incoming solar radiation) are the most likely cause of large-scale changes in Earth's climate during the Quaternary. In other words, variations in the intensity and timing of heat from the sun are the most likely cause of the glacial/interglacial cycles. This solar variable was neatly described by the Serbian scientist, Milutin Milankovitch, in 1938. There are three major components of the Earth's orbit about the sun that contribute to changes in our climate. First, the Earth's spin on its axis is wobbly, much like a spinning top that starts to wobble after it slows down. This wobble amounts to a variation of up to 23.5 degrees to either side of the axis. The amount of tilt in the Earth's rotation affects the amount of sunlight striking the different parts of the globe. The greater the tilt, the stronger the difference in seasons (i.e., more tilt equals sharper differences between summer and winter temperatures). The range of motion in the tilt (from left-of-center to right-of-center and back again) takes place over a period of 41,000 years. As a result of a wobble in the Earth's spin, the position of the Earth on its elliptical path changes, relative to the time of year. This phenomenon is called the precession of equinoxes. The cycle of equinox precession takes 23,000 years to complete. In the growth of continental ice sheets, summer temperatures are probably more important than winter.
How does the ice build up?
Throughout the Quaternary period, high latitude winters have been cold enough to allow snow to accumulate. It is when the summers are cold, (i.e., summers that occur when the sun is at its farthest point in Earth's orbit), that the snows of previous winters do not melt completely. When this process continues for centuries, ice sheets begin to form. Finally, the shape of Earth's orbit also changes. At one extreme, the orbit is more circular, so that each season receives about the same amount of insolation. At the other extreme, the orbital ellipse is stretched longer, exaggerating the differences between seasons. The eccentricity of Earth's orbit also proceeds through a long cycle, which takes 100,000 years. Major glacial events in the Quaternary have coincided when the phases of axial tilt, precession of equinoxes and eccentricity of orbit are all lined up to give the northern hemisphere the least amount of summer insolation.
What makes the ice melt when the glaciation is over?
Major interglacial periods have occurred when the three factors line up to give the northern hemisphere the greatest amount of summer insolation. The last major convergence of factors giving us maximum summer warmth occurred 11,000 years ago, at the transition between the last glaciation and the current interglacial, the Holocene. During the late Pleistocene, the Rocky Mountain regions of Canada and the regions farther west were almost engulfed in the Cordilleran Ice Sheet, while most of Canada east of the Rockies and the north-central and northeastern United States were covered by the Laurentide Ice Sheet. The divide between the two ice sheets lay east of the Rockies, with the two ice bodies meeting near the U.S.-Canadian border in eastern Montana. The Laurentide ice sheet is thought to have been as much as two miles thick at the center.
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